76 



ISLAND LIFE. 



[part I. 



restriction to limited areas; and it is only by bearing these 

 considerations in mind that we can find a satisfactory ex- 

 planation of the many anomalies we meet with in studying 

 their distribution. 



The Dispersal of Land Mollusca. — The only other group 

 of animals we need now refer to is that of the air-breathing 

 mollusca, commonly called land-shells. These are almost as 

 ubiquitous as insects, though far less numerous ; and their wide 

 distribution is by no means so easy to explain. The genera 

 have usually a very wide, and often a cosmopolitan, range, while 

 the species are rather restricted, and sometimes wonderfully so. 

 Not only do single islands, however small, often possess peculiar 

 species of land-shells, but sometimes single mountains or valleys, 

 or even a particular mountain side, possess species or varieties 

 found nowhere else upon the globe. It is pretty certain that 

 they have no means of passing over the sea but such as are very 

 rare and exceptional. Some which possess an operculum, or 

 which close the mouth of the shell with a diaphragm of secreted 

 mucus, may float across narrow arms of the sea, especially 

 when protected in the crevices of logs of timber ; while in the 

 young state when attached to leaves or twigs they may be 

 carried long distances by hurricanes.^ Owing to their exceedingly 

 slow motion, their powers of voluntary dispersal, even on land, 

 are very limited, and this will explain the extreme restriction of 

 their range in many cases. 



Great Antiquity of Land-Shells. — The clue to the almost 

 universal distribution of the several families and of many genera, 

 is to be found, however, in their immense antiquity. In the 

 Pliocene and Miocene formations most of the land-shells are 

 either identical with living species or closely allied to them, 



1 Mr. Darwin found that the large Helix pomatia lived after immersion 

 in sea-water for twenty days. It is hardly likely that this is the extreme 

 limit of their powers of endurance, but even this would allow of their being 

 floated many hundred miles at a stretch, and if we suppose the shell to be 

 partially protected in the crevice of a log of wood, and to be thus out of 

 water in calm weather, the distance might extend to a thousand miles or 

 more. The eggs of fresh-water mollusca are known to attach themselves 

 to the feet of aquatic birds, and this is supposed to account for their very 

 wide diffusion. 



